For field identification, I think there is no substitute for a book. I would recommend Guide des Sauterelles, Grillons et Criquets d"Europe occidentale by Bellmann and Luquet.
However, there is a key to the French species of Acrotylus in this paper: http://inpn.mnhn.fr/fichesEspece/Acrotylus_braudi.pdf You would need to have specimens in the hand to use it, I think.